1. Field of the Invention
This invention relates to the polishing of a surface of alpha-alumina, commonly referred to as sapphire.
2. Prior Art
Prior art methods for the polishing of an alpha-alumina surface include mechanical processes whereby the alpha-alumina is shaped by cutting with a diamond-impregnated saw blade, and polished with successively finer grades of diamond or carborundum powders. However, diamond-polished surfaces carry work-damage to a depth at least equal to the size of the diamond particle used in the polish. Damage may even extend beneath the alpha-alumina surface.
When alpha-alumina is used as an insulating substrate in electronic applications such as semiconductor devices, transistors or microcircuits, the quality of the final product depends mainly on the characteristics of the semiconductor film which, in turn, depends directly on the flatness and damage-free qualities of the substrate.
The mechanically polished alpha-alumina surface may then be finished by "healing" the surface damage in an annealing process at temperatures above 1200.degree. C in hydrogen. Alternatively, the mechanical damage may be removed chemically. Chemical polishing has been reported in the British Journal of Applied Physics, Volume 18, pages 1357-1382 (1967) in an article entitled, "Single-Crystal Films of Silicon on Insulators", by J. D. Filby and S. Nielsen. The chemical methods involve etching with molten materials such as lead fluoride, potassium persulphate in boric oxide, borax, anhydrous HCl, sulfur fluorides and silicon vapors. However, these processes require high temperatures such as 1000.degree. C for borax and 1400.degree. C for anhydrous HCl. Further, the additional disadvantage of the formation of a residue which is difficult to remove has been reported when using borax.
An example of chemical etching is found in U.S. Pat. No. 3,753,775 which utilizes borax vapors at temperatures between 1000.degree. C and 1200.degree. C. A significant disadvantage of this process is the necessity of providing materials for the apparatus which will not react with borax at high temperatures. Thus, the material suggested in this method is platinum, the use of which constitutes a significant economic disadvantage.